Field of the Invention
The invention relates to an apparatus for applying printing ink onto a form roller, preferably for flexographic and gravure presses, as well as for planographic presses, using an ink fountain. More particularly, the invention is directed to an ink feed system which can transport and distribute even heavy ink from an ink tank via an enclosed ink fountain to the form roller, wherein the degree of viscosity prevailing during the transfer from the enclosed ink fountain onto the form roller can be adjusted by the rotational speed of the transfer roller located in the enclosed ink fountain. Moreover, the pressure as well as the flow rate in the ink fountain can be controlled by adjusting the rotational velocity of a pump used in connection with the ink supply.
Form rollers are conventionally known as the ink rollers which contact and supply ink to the printing form is a letter press machine or to the press plate in a lithographic or a rotary press. It is common practice to distribute printing inks to form rollers, for example, of flexographic and gravure presses, by immersing duct rollers or feed rollers into a color duct and pressing the accepted ink film against the form roller, the overflowing ink returning to the color duct. Form rollers are also known in which, in addition to the reservoir in the color duct, an auxilliary reservoir is employed outside the color duct and in which an ink pump is provided that pumps the printing ink continuously into the color duct, with the ink returning by the force of gravity. Other form rollers are known, as exemplified by U.S. Pat. No. 3,766,856 and published West German Application No. 31 35 711, in which the printing ink is fed to the form roller through a distribution chamber or through a guide conduit. Here, too, however, the overflowing ink returns by gravitational force to a reservoir provided with an ink pump. In these designs, the ink viscosity is known to be changed by the addition of solvents.
Depending upon the printing quality requirements, the solvents are admixed with the printing ink directly in the color duct or in an ink tank which is connected to the color duct via a return hose coupling, whereby a pump in the ink tank maintains a continuous ink flow between the color duct and ink tank. High quality and levelness requirements can be achieved with this type of circulating ink system and with an auxiliary viscosity regulator. However, the energy expended for this type of prior art equipment for the evaporation of the solvents and the attendant environmental pollution, as well as the expensive cleaning of the circulating ink system for the change of inks, prove to be significant drawbacks. This is especially true since a low viscosity for these circulating ink systems is not necessary for obtaining good printing qualities.
It is also known in the prior art to provide form rollers for planographic presses in which inks with a high viscosity are friction-glazed and, by means of a set of distributor rollers, distributed evenly across the form roller. A stationary grinding effect, however, can only be obtained by a tandem connection of a plurality of rollers.
A principal object of the invention is to provide an apparatus for applying printing ink onto a form roller that can evenly transfer heavy inks without dilution onto the form roller, e.g., an engraved roller.
It is a further object to provide such an apparatus in which the ink viscosity can be adjusted just before the ink transfer onto the form roller by friction in an enclosed ink chamber or fountain using a thin friction clearance between the convex surface of a transfer or friction roller and a concave cylindrical surface of the fountain housing.
Yet a further object is to provide the ink-loading spaces in the ink supply system so that, when exchanging inks, washing with a relatively small amount of solvent will lead to thorough cleaning.
These and other objects are achieved by the teachings of the invention, wherein an apparatus for applying printing ink onto a form roller of a printing machine comprises an enclosed ink fountain mounted adjacent the form roller and having a housing with a chamber formed therein; and a friction roller rotatably and eccentrically moveable within the chamber of the ink fountain housing, wherein the eccentricity distance of the friction roller in relationship to the ink fountain housing and the rotational direction and speed of the friction roller are independently adjustable.
An ink pump is used with a preferably air-tight ink tank. This ink pump conveys the printing ink under pressure from the pump outlet, via a friction chamber or an enclosed ink fountain and an enclosed ink distribution chamber, to the form roller. A centrifugal pump can likewise be used as a gear pump or a hose pump for supply of the ink. In all cases, pump pressure and flow rate can be controlled by adjusting the pump's rotational speed.
The ink is conveyed to an ink fountain through an inlet opening connected to a pump supply pipe. Since the ink flow rate is solely determined by the ink consumption, the pipe cross section can have a diameter of only a few millimeters.
In the ink fountain, which is preferably cylindrical but which may also have an oval or a polygonal cross section, there is povided a transfer or friction roller which is movably mounted and pivoted about its longitudinal axis and whose radial distance to the inner chamber wall of the ink fountain can be adjusted.
Opposite the ink supply inlet or at an angle thereto, there is an ink outlet opening which leads to a narrow ink distribution chamber into which is fed the ink that has been adjusted by friction in the ink fountain to a specific viscosity. This ink distribution chamber is sealed on all sides and is defined by two doctor blades in the direction of rotation of the form roller.
The adjustment of the viscosity of the printing ink by the transfer roller presupposes a controllable speed of rotation of the transer roller and adjustable radial distance of the transfer roller to the inner chamber wall of the ink fountain independent of the circumferential velocity of the form roller due to the effect of the two "influencing variables"--shearing force and temperature. The shearing force can be determined within a broad range by changing the circumferential speed of the transfer roller and by varying the friction clearance (i.e., the distance "d" between the transfer roller and the inner chamber wall of the ink fountain on the ink outlet side).
Due to the thermoplastic conditions prevailing in almost all flexographic and gravure printing inks, the resultant temperature as a function of the shearing force also affects the viscosity.
The present test results with heavy links show that in order to obtain good printing qualities, it is not necessary to lower the ink viscosity to a conventional value of about 25 seconds in a Ford cup having an efflux with a diameter of 4 mm.
In one test, a water-soluble ink was used with an efflux time of 56 seconds from a 6-mm Ford cup. If the 50-mm transfer roller had a rotational speed of 350 rpm, this ink could be pressed in the doctor blade chamber with the proper quality.
The thixotrope conditions of the ink was evidenced by a treatment in an agitator. After stirring 30 seconds, the viscosity dropped from 56 seconds to 21 seconds efflux time.
However, the requirement for a substantially aqueous liquid is ascribable to the construction of the conventional form rollers in which the ink circulation is maintained by the force of gravity of the ink. A free return of the ink used in the above example is not possible because it leads to a back-pressure, as demonstrated by tests with a conventional form roller. A throwing out of ink lumps from the color duct was observed.
The pressure prevailing in the ink fountain can be adjusted by varying the rotational speed of the pump with coincidental control of the thickness of the ink layer on the form roller.
Since there is no external ink circulation, the amount of ink fed can only flow off via the form roller. Thus, the amount of ink per pressure report can be fixed.
With the apparatus for applying print ink onto a form roller of the present invention and with a forced supply of the ink under adjustable pressure, heavy inks can be used and thereby, a greatly reduced percentage of solvent. This leads to advantages because of the reduction of the energy for drying and reducing the solvent vapor emission.
Other advantages of the apparatus of the invention are seen in the fact that the reaction time from the display of a change of tone value in the print image to the change of color viscosity can be reduced to a few seconds. This permits a tone value correction at the rpm of the transfer roller as a correction variable. The rotational speed of the ink pump and, thereby, of the filling pressure can also be utilized as a correcting variable.
This type of apparatus also admits of a reduction of the entire ink-loading spaces to a lower value of up to one to two powers of 10 in comparison to ink circulating systems of the prior art, and thereby enables cleaning by rinsing without the necessity of dismounting the color duct that would otherwise be necessary.
Since the ink circulation occurs within the ink fountain, only a relatively small volume of ink is in the ink circulating spaces. Also with the apparatus of the invention, in order to prevent the ink from drying during relatively long idle periods, solvents can be fed. Solvent delivery can be effected directly into the ink fountain in a manner known from the prior art, controlled by the driving movement of the transfer roller, or it can be effected by spraying an appropriate solvent directly onto the form roller. Another possibility of adding solvent in the case of relatively long idle periods consists of using one of the ink supply ducts or the return ducts from which the ink is supplied and returned to the ink tank. In this case, the ink tank volume again is determined only by the effective consumption of quantities per job.